1. Field of the Invention
The field of this invention lies within the cable anchoring art. More particularly, it lies within the field of anchoring or snubbing such cables around a drum wherein the cable is utilized for oil well drilling functions.
The tension on the cable is measured through the rotative movement of the drum by means of a load cell which provides a reading as well as an anchoring thereof.
2. The Prior Art
The prior art related to cable anchor weighing devices, and load testing deadline anchors for oil well derricks, generally comprises U.S. Pat. Nos. issued to E. L. Decker, 24,092, 2,807,957, 2,984,103, and 3,045,480; to J. D. Spalding, 2,488,070; and, to F. L. Gillett, 3,056,287.
The foregoing prior art generally incorporates various features in order to anchor, as well as determine the tension on a line.
One of the more popular cable anchors is that patented by Gillett, U.S. Pat. No. 3,056,287. Generally, this cable anchor incorporates what is referred to as a snubbing drum which is rotatively mounted on a stub shaft or spindle by means of axially spaced bearings. The drum is provided with axially spaced radial flanges at its outer periphery and an inner tapered snubbing surface. A weigh beam or torque arm is attached to the drum at one end and carries a releasable cable clamp at its opposite end. The releasable cable clamp is tangentially aligned with the snubbing surface of the drum. A cable or line extending from a storage reel passes through the clamp, is wrapped several times around the drum, and passes upwardly to the hoisting blocks.
The clamping means can be released so that increased amounts of cable from the storage reel can be let out and placed around the snubbing surface of the drum.
The cable anchor is also connected to what is referred to as a load cell. The load cells are generally hydraulic load cells, but can be in other forms responsive to movement or rotation of the snubbing drum.
The load cell is located between the torque arm and the anchor base. Weight or tension on the cable causes movement of the drum. This movement is transmitted to the load cell through the torque arm.
The foregoing hydraulic load cells are connected so that movement of the torque arm creates a positive amount of pressure in a hydraulic line. A Bourdon tube in fluid connected relationship to a hydraulic pressure line of the load cell and a meter provides a direct reading of pressure changes corresponding to load changes in the cable.
A substantial drawback of the prior art is that the foregoing drums or cable anchors, such as that shown in the Gillett patent, are cast or made of materials that are quite expensive. This invention overcomes the requirement for a cast structure in the drum by virtue of its unique design. The invention incorporates a series of radial arms that form the rigidifying spokes of the drum in an optimum manner in combination with a weigh beam configuration. A superior anchor is provided, resulting in improved performance to the cable anchor and cable load measuring system.
In particular, the invention provides a unique weigh beam or torque arm configuration which is superior to that of the prior art. The weigh beam or arm is in the form of a triangularly braced weigh beam that is in offset relationship with regard to the mounting point of the drum. That is, the bore through the drum for attachment to the stub shaft is offset with regard to the axial center of the drum to allow for an improved movement of the weigh beam with respect to the load cell.
The weigh beam of the invention is braced with a triangular bracing element that extends from the periphery or circumferential edge of one portion of the drum to the extremities of the weigh beam. This provides a dual bracing for the drum, as well as the weigh beam, and partially allows for a welded drum to be utilized. The placement of the drum and weigh beam on the supporting structure allows for an optimum readout by the weigh beam.
In addition to the foregoing features of this invention, the position of the weigh beam and the geometrical movement described thereby is substantially enhanced over the prior art. In the prior art, the torque arm or weigh beam, moved through an arc or radius which did not uniformly reflect the analog, in a geometrical sense, of the actual movement or corresponding tension applied to the cable around the drum. It has been found that the end of the weigh beam of this invention describes a movement that corresponds to a direct parallelogram movement of the entire device. A more uniform and even movement of the weigh beam is thus provided corresponding to the tension on the cable, resulting in more accurate response by the load cell and readout thereon.
As a consequence, this invention is a substantial improvement as far as structural characteristics and performance over the prior art.